so perfect, that the expectation of actual agreement in the admeasurement to be obtained from them, naturally arose; but in this I was disappointed on finding them to vary on the one angle from 101°. 25′ to 101°. 51′; and on the other, from 78°. 10′ to 78°. 28′.
This want of co-incidence in the admeasurements obtained from crystals which, on account of their brilliancy, seemed unexceptionable, induced me to submit to the reflecting goniometer some cleavages made in the direction of the primitive planes, and selected with the utmost care. Six of these agreed perfectly in affording on each of the acute angles of each fragment the incidence of 70°. 18′, and on each of the obtuse angles that of 101°. 42′; the former being 9′. 47″ less, and the latter, in being 9′. 47″ more, than the value assigned to these angles by Haüy.
This remarkable agreement afforded by the cleavages of the sulphate of barytes, induces regret that it is not in like manner practicable to obtain regular fractures of quartz. The evidence above cited in regard to the difficulties attending the measurement of that substance, by means of the reflections afforded by the natural planes of its crystals, tend, it must be owned, to render it somewhat uncertain whether the only co-incidences obtained, viz. those 94°. 15′ 85°. 45′, are the true angles of its primitive rhomboid; but the co-incidences given by the fracture of the sulphate of barytes, seem to authorize the conclusion that the true value of the angles of its primitive crystals are 78°. 18′ and 101°. 42′.
It would not perhaps be reasonable to draw, from evidence relating only to two substances, the inference that the actual value of the angles of mineral substances in the general cannot by any means be obtained by admeasurements attempted upon the natural planes of their crystals. The difficulties attendant on many of them will be found to be very great; in regard to some they seem insurmountable.
